Understanding plant reproductive diversity

Self-Pollinated Types and Ecological Adaptations of the Desert Plant Gymnocarpos przewalskii

In desert plants, outcrossing is frequently limited by pollinator scarcity, leading to a certain percentage of self-fertilization. However, research on the ecological adaptations of self-fertilized seeds remains limited. Gymnocarpos przewalskii Maxim, a Tertiary relict plant in the arid deserts of Northwest China, exhibits pronounced self-pollination. In this study, the population of G. przewalskii from the fifth regiment of Alar City was selected to explore its self-pollination types, self-pollination percentages, and ecological adaptations. We found that artificially cross-pollinated G. przewalskii produced heavier seeds, faster germination, seedlings with greater biomass, and stronger environmental adaptability than self-pollination. However, the frequency of insect visits per flower was less than one. The fruit setting rate of natural pollination was 6.90%, while that of self-pollination was 4.43%, accounting for 64.20% of the natural fruit setting rate. Additionally, G. pzewalskii's filaments elongated rapidly to make the anthers and stigma at the same height before flowering. These characteristics suggest that G. przewalskii is capable of autonomous self-pollination and is prior selfing. Gymnocarpos przewalskii likely produces a high proportion of the selfing merely to ensure population survival. These findings offer valuable insights into the adaptation of desert plants to the scarcity of pollinators.

New QTLs involved in the control of stigma position in tomato

BACKGROUND

Tomato mating systems were strongly affected by domestication events. Mutations disrupting self-incompatibility paralleled by changes retracting the stigma position (SP) within the staminal cone conferred strict autogamy and self-fertility to the cultivated forms. Although major genes affecting these changes have been identified, SP control in domesticated forms that retain a constitutive or heat-inducible noninserted SP needs elucidation. To widen the possibility of identifying SP genetic determinants, we analyzed the trait in four populations (two germplasm collections, a multiparental recombinant inbred and a biparental progeny) under different environmental conditions (normal and heat stressed).

RESULTS

Overall, 37 markers significantly associated with the trait were identified. Several colocalizations were found, both among regions first reported in this work and among them and previously reported positions. This finding supported the reliability of the analysis. Three such regions, in the long arms of chromosomes 1, 8 and 11, were validated in an independent segregating population, and candidate genes in confidence intervals were identified among transcription factors and hormone-, stress- and cell wall-related genes.

CONCLUSION

Overall, this work supported the hypothesis that the SP phenotype is controlled by different key genes in tomato, paving the way for the identification of novel players and novel mechanisms involved in the regulation of herkogamy.

Loss of pollinator diversity consistently reduces reproductive success for wild and cultivated plants

Pollination is a crucial ecosystem service, yet pollinator species diversity is declining as a result of factors such as climate change, habitat loss and agricultural intensification. While previous studies have often examined the extreme scenario of complete pollinator removal, showing negative impacts on plant reproductive success, we take a more realistic approach by focusing on the effects of decreasing pollinator diversity. Our global meta-analysis reveals a notable negative impact of reduced pollinator species diversity on plant reproductive success measures, such as seed set, fruit set and fruit weight. Notably, this effect varies across plant families, impacting both self-incompatible and self-compatible species. We also find that wild plant species suffer more than cultivated ones. Furthermore, the loss of invertebrate, nocturnal and wild pollinators has a more substantial impact than the loss of vertebrate, diurnal or managed pollinators. Overall, our findings consistently underscore the positive role of biodiversity in maintaining ecosystem functioning, highlighting the urgency of mitigating factors that lead to the decline in pollinator species diversity.

Ancient structural variants control sex-specific flowering time morphs in walnuts and hickories

Balanced mating type polymorphisms offer a distinct window into the forces shaping sexual reproduction strategies. Multiple hermaphroditic genera in Juglandaceae, including walnuts (Juglans) and hickories (Carya), show a 1:1 genetic dimorphism for male versus female flowering order (heterodichogamy). We map two distinct Mendelian inheritance mechanisms to ancient (>37 million years old) genus-wide structural DNA polymorphisms. The dominant haplotype for female-first flowering in Juglans contains tandem repeats of the 3' untranslated region of a gene putatively involved in trehalose-6-phosphate metabolism and is associated with increased cis gene expression in developing male flowers, possibly mediated by small RNAs. The Carya locus contains ~20 syntenic genes and shows molecular signatures of sex chromosome-like evolution. Inheritance mechanisms for heterodichogamy are deeply conserved, yet may occasionally turn over, as in sex determination.

Floral traits underlying mating system differentiation in the wind-pollinated sister species Oryza rufipogon and Oryza nivara

The shift from outcrossing to predominantly selfing is one of the most common transitions in plant evolution. This evolutionary shift has received considerable attention from biologists; however, this work has almost exclusively been focused on animal-pollinated systems. Despite the seminal ecological and economic importance of wind-pollinated species, the mechanisms controlling the degree of outcrossing in wind-pollinated taxa remain poorly understood. As a first step toward addressing this issue, we have conducted a comparative study of floral biology between two recently diverged sister species, Oryza rufipogon and Oryza nivara (Poaceae), that are wind-pollinated and possess distinct mating systems with O. rufipogon being outcrossing and O. nivara highly self-fertilized Therefore, these species present an ideal system for exploring mating system evolution in wind-pollinated taxa. We have identified key floral traits that differ between populations of these species and that are associated with mating system divergence including anther length, anther basal pore size, stigma papillae density, panicle shape, panicle exsertion, pollen viability, and early anther dehiscence. Of these traits, large anther basal pore size and early anther dehiscence are hypothesized to confer reliable autogamous selfing in O. nivara. Manipulations of floret number were conducted to partition the role of geitonogamy and autogamy in conferring self-fertilization. This experiment revealed that selfing in O. nivara is consistent with autogamous selfing, whereas O. rufipogon achieves selfing through geitonogamy. This study serves as a model for understanding the floral mechanisms controlling the outcrossing rate in other wind-pollinated systems, most notably other grasses.

Maintenance of andromonoecy in an autogamous species: Superior male function in male flowers of the endangered Sagittaria guayanensis

Andromonoecy is a rare sexual system in plants. The function of additional male flowers in andromonoecious species has been widely discussed; however, few studies have taken offspring fitness into account. In addition, little is known about the mechanisms that maintain andromonoecy in autogamous species. In this study, we compared morphology, pollinator preference, pollen production and export, siring ability, natural siring success, hundred seed dry weight, and seed germination rates between male and hermaphroditic flowers in an endangered autogamous andromonoecious species, Sagittaria guayanensis. Male flowers, which are larger than hermaphroditic flowers, required fewer resources to produce. Pollinators visited male flowers more frequently than they visited hermaphroditic flowers. In addition, pollen production and export were higher in male flowers. Hand pollination demonstrated that siring ability did not differ between flower type. However, the natural siring success of male flowers was triple that of hermaphroditic flowers. The seeds sired by male flowers performed better than those sired by hermaphroditic flowers, with greater dry weight and higher germination rate. In conclusion, male flowers may be superior pollen donors for outcrossing. The maintenance of andromonoecy in S. guayanensis may result from the better performance of male flowers in male function compared to that of hermaphroditic flowers.

The interplay between climatic niche evolution, polyploidy and reproductive traits explains plant speciation in the Mediterranean Basin: a case study in Centaurium (Gentianaceae)

Speciation and diversification patterns in angiosperms are frequently shaped by niche evolution. Centaurium Hill is a Mediterranean genus with ca. 25 species, of which 60% are polyploids (tetra- and hexaploids), distributed mainly in the Mediterranean Basin and in areas with temperate and arid climates of Asia, Europe, North-Central Africa and North America. The evolutionary history of this genus has been studied using morphological, biogeographical and molecular approaches, but its climatic niche characterization and its relation with genome evolution (chromosome number and ploidy level) has not been addressed yet. Thus, this study aims to identify the role of the evolution of climatic niche, ploidy level, life cycle and floral traits in the diversification of Centaurium. Climatic niche characterization involved estimating present climate preferences using quantitative data and reconstructing ancestral niches to evaluate climatic niche shifts. The evolution of climatic niche towards selective optima determined by ploidy level (three ploidy levels) and different binary traits (polyploidy, floral size, floral display, herkogamy and life cycle) was addressed under the Ornstein-Uhlenbeck model. Chromosome number evolution was inferred using the ChromoSSE model, testing if changes are clado- or anagenetic. Chromosome number evolution and its link with cladogenesis, life cycle and floral traits was modeled on the phylogeny. The reconstruction of the ancestral niches shows that Centaurium originated in a mild climate and diversified to both humid and cold as well as to dry and warmer climates. Niche conservatism was estimated in the climatic niche of the ancestors, while the climatic niche of the current taxa experienced transitions from their ancestors' niche. Besides, the evolution of climatic niche towards multiple selective optima determined by the studied traits was supported, life cycle optima receiving the highest support. The reconstruction of chromosome number transitions shows that the rate of speciation process resulting from chromosomal changes (chromosomal cladogenesis) is similar to that of non-chromosomal cladogenesis. Additionally, dependent evolution of floral size, floral display and herkogamy with chromosome number variation was supported. In conclusion, polyploidization is a crucial process in the Mediterranean region that assisted speciation and diversification into new areas with different climates, entailing niche shifts and evolution of reproductive strategies.

Multiple Horizontal Mini-chromosome Transfers Drive Genome Evolution of Clonal Blast Fungus Lineages

Crop disease pandemics are often driven by asexually reproducing clonal lineages of plant pathogens that reproduce asexually. How these clonal pathogens continuously adapt to their hosts despite harboring limited genetic variation, and in absence of sexual recombination remains elusive. Here, we reveal multiple instances of horizontal chromosome transfer within pandemic clonal lineages of the blast fungus Magnaporthe (Syn. Pyricularia) oryzae. We identified a horizontally transferred 1.2Mb accessory mini-chromosome which is remarkably conserved between M. oryzae isolates from both the rice blast fungus lineage and the lineage infecting Indian goosegrass (Eleusine indica), a wild grass that often grows in the proximity of cultivated cereal crops. Furthermore, we show that this mini-chromosome was horizontally acquired by clonal rice blast isolates through at least nine distinct transfer events over the past three centuries. These findings establish horizontal mini-chromosome transfer as a mechanism facilitating genetic exchange among different host-associated blast fungus lineages. We propose that blast fungus populations infecting wild grasses act as genetic reservoirs that drive genome evolution of pandemic clonal lineages that afflict cereal crops.

Intraspecific variation in pollination ecology due to altitudinal environmental heterogeneity

Plant-pollinator interactions are constrained by floral traits and available pollinators, both of which can vary across environmental gradients, with consequences for the stability of the interaction. Here, we quantified how the pollination ecology of a high-mountain hummingbird-pollinated plant changes across a progressively more stressful environmental gradient of the Venezuelan Andes. We compared pollination ecology between two populations of this plant: Piedras Blancas (PB) and Gavidia (GV), 4450 and 3600 m asl, respectively. We hypothesised that self-compatibility might be higher at the higher altitude site, however we found that flowers showed similar capacities for self-compatibility in both localities. Seed production by flowers exposed to natural pollinators was significantly higher in the lower locality, where we also found higher nectar quality, larger flowers and increased frequencies of pollinator visitations. Interestingly, the population energy offered in the nectar was the same for both localities due to the higher density and floral aggregation found in the higher altitude population. Our study demonstrates how two plant populations in different environmental conditions have different pollination ecology strategies. Pollinator visitations or their absence result in trait associations in one population that are independent in the other. These population differences are not explained by differences in pollinator assembly, but by environmental heterogeneity.

Genetic Causes and Genomic Consequences of Breakdown of Distyly in Linum trigynum

Distyly is an iconic floral polymorphism governed by a supergene, which promotes efficient pollen transfer and outcrossing through reciprocal differences in the position of sexual organs in flowers, often coupled with heteromorphic self-incompatibility. Distyly has evolved convergently in multiple flowering plant lineages, but has also broken down repeatedly, often resulting in homostylous, self-compatible populations with elevated rates of self-fertilization. Here, we aimed to study the genetic causes and genomic consequences of the shift to homostyly in Linum trigynum, which is closely related to distylous Linum tenue. Building on a high-quality genome assembly, we show that L. trigynum harbors a genomic region homologous to the dominant haplotype of the distyly supergene conferring long stamens and short styles in L. tenue, suggesting that loss of distyly first occurred in a short-styled individual. In contrast to homostylous Primula and Fagopyrum, L. trigynum harbors no fixed loss-of-function mutations in coding sequences of S-linked distyly candidate genes. Instead, floral gene expression analyses and controlled crosses suggest that mutations downregulating the S-linked LtWDR-44 candidate gene for male self-incompatibility and/or anther height could underlie homostyly and self-compatibility in L. trigynum. Population genomic analyses of 224 whole-genome sequences further demonstrate that L. trigynum is highly self-fertilizing, exhibits significantly lower genetic diversity genome-wide, and is experiencing relaxed purifying selection and less frequent positive selection on nonsynonymous mutations relative to L. tenue. Our analyses shed light on the loss of distyly in L. trigynum, and advance our understanding of a common evolutionary transition in flowering plants.

Flower opening dynamics, pollen-ovule ratio, stigma receptivity and stigmatic pollen germination (in-vivo) in Chaenomeles speciosa (Sweet) Nakai

Although Chaenomeles is widely used in horticulture, traditional Chinese medicine and landscape greening, insufficient research has hindered its breeding and seed selection. This study investigated the floral phenology, floral organ characteristics, palynology, and breeding systems of Chaenomeles speciosa (Sweet) Nakai. The floral characteristics of C. speciosa were observed both visually and stereoscopically. The microstructures of the flower organs were observed using scanning electron microscopy. Pollen stainability was determined using triphenyl tetrazolium chloride staining. Stigma receptivity was determined using the benzidine-H2O2 method and the post-artificial pollination pollen germination method. The breeding system was assessed based on the outcrossing index and pollen-ovule ratio. The flowers of C. speciosa were bisexual with a flowering period from March to April. The flowering periods of single flowers ranged from 8 to 19 d, and those of single plants lasted 18-20 d. The anthers were cylindrical, with the base attached to the filament, and were split longitudinally to release pollen. The flower had five styles, with a connate base. The ovaries had five carpels and five compartments. The inverted ovules were arranged in two rows on the placental axis. The stigma of C. speciosa was dry and had many papillary protrusions. In the early flowering stage (1-2 d of flowering), the pollen exhibited high stainability (up to 84.24%), but all stainability was lost at 7 d of flowering. Storage at - 20 °C effectively delayed pollen inactivation. The stigma receptivity of C. speciosa lasted for approximately 7 days, and the breeding system was classified as outcrossing with partial self-compatibility.

Patterns and drivers of plant sexual systems in the dry-hot valley region of southwestern China

Sexual systems play important roles in angiosperm evolution and exhibit substantial variations among different floras. Thus, studying their evolution in a whole flora is crucial for understanding the formation and maintenance of plant biodiversity and predicting its responses to environmental change. In this study, we determined the patterns of plant sexual systems and their associations with geographic elements and various life-history traits in dry-hot valley region of southwestern China, an extremely vulnerable ecosystem. Of the 3166 angiosperm species recorded in this area, 74.5% were hermaphroditic, 13.5% were monoecious and 12% were dioecious, showing a high incidence of diclinous species. Diclinous species were strongly associated with tropical elements, whereas hermaphroditic species were strongly associated with temperate and cosmopolitan elements. We also found that hermaphroditism was strongly associated with showy floral displays, specialist entomophily, dry fruits and herbaceous plants. Dioecy was strongly associated with inconspicuous, pale-colored flowers, generalist entomophily, fleshy fruits, and woody plants, whereas monoecy was strongly associated with inconspicuous, pale-colored flowers, anemophily, dry fruits, and herbaceous plants. In addition, hermaphroditic species with generalist entomophily tended to flower in the dry season, whereas diclinous species with specialist entomophily tended to flower in the rainy season. However, independent of sexual systems, plants that produce dry fruits tended to flower in the rainy season and set fruits in the dry season, but the opposite pattern was found for fleshy fruit-producing plants. Our results suggest that in the dry-hot valleys, plant sexual systems are associated with geographic elements as well as various life-history traits that are sensitive to environmental change.

Conservation and diversity of the pollen microbiome of Pan-American maize using PacBio and MiSeq

Pollen is a vector for diversification, fitness-selection, and transmission of plant genetic material. The extent to which the pollen microbiome may contribute to host diversification is largely unknown, because pollen microbiome diversity within a plant species has not been reported, and studies have been limited to conventional short-read 16S rRNA gene sequencing (e.g., V4-MiSeq) which suffers from poor taxonomic resolution. Here we report the pollen microbiomes of 16 primitive and traditional accessions of maize (corn) selected by indigenous peoples across the Americas, along with the modern U.S. inbred B73. The maize pollen microbiome has not previously been reported. The pollen microbiomes were identified using full-length (FL) 16S rRNA gene PacBio SMRT sequencing compared to V4-MiSeq. The Pan-American maize pollen microbiome encompasses 765 taxa spanning 39 genera and 46 species, including known plant growth promoters, insect-obligates, plant pathogens, nitrogen-fixers and biocontrol agents. Eleven genera and 13 species composed the core microbiome. Of 765 taxa, 63% belonged to only four genera: 28% were Pantoea, 15% were Lactococcus, 11% were Pseudomonas, and 10% were Erwinia. Interestingly, of the 215 Pantoea taxa, 180 belonged to a single species, P. ananatis. Surprisingly, the diversity within P. ananatis ranged nearly 10-fold amongst the maize accessions analyzed (those with ≥3 replicates), despite being grown in a common field. The highest diversity within P. ananatis occurred in accessions that originated near the center of diversity of domesticated maize, with reduced diversity associated with the north-south migration of maize. This sub-species diversity was revealed by FL-PacBio but missed by V4-MiSeq. V4-MiSeq also mis-identified some dominant genera captured by FL-PacBio. The study, though limited to a single season and common field, provides initial evidence that pollen microbiomes reflect evolutionary and migratory relationships of their host plants.

A Foundational Population Genetics Investigation of the Sexual Systems of Solanum (Solanaceae) in the Australian Monsoon Tropics Suggests Dioecious Taxa May Benefit from Increased Genetic Admixture via Obligate Outcrossing

Solanum section Leptostemonum is an ideal lineage to test the theoretical framework regarding proposed evolutionary benefits of outcrossing sexual systems in comparison to cosexuality. Theoretically, non-cosexual taxa should support more genetic diversity within populations, experience less inbreeding, and have less genetic structure due to a restricted ability to self-fertilize. However, many confounding factors present challenges for a confident inference that inherent differences in sexual systems influence observed genetic patterns among populations. This study provides a foundational baseline of the population genetics of several species of different sexual systems with the aim of generating hypotheses of any factor-including sexual system-that influences genetic patterns. Importantly, results indicate that dioecious S. asymmetriphyllum maintains less genetic structure and greater admixture among populations than cosexual S. raphiotes at the same three locations where they co-occur. This suggests that when certain conditions are met, the evolution of dioecy may have proceeded as a means to avoid genetic consequences of self-compatibility and may support hypotheses of benefits gained through differential resource allocation partitioned across sexes. Arguably, the most significant finding of this study is that all taxa are strongly inbred, possibly reflective of a shared response to recent climate shifts, such as the increased frequency and intensity of the region's fire regime.

Age-specific allocation of glucosinolates within plant reproductive tissues

The Optimal Defense Theory (ODT) predicts that the distribution of defenses within a plant should mirror the value and vulnerability of each tissue. Although the ODT has received much experimental support, very few studies have examined defense allocation among reproductive tissues and none assessed simultaneously how these defenses evolve with age. We quantified glucosinolates in perianths, anthers and pistils at different bud maturity stages (i.e., intermediate flower buds, old flower buds and flowers) of undamaged and mechanically damaged plants of an annual brassicaceous species. The youngest leaf was used as a reference for vegetative organs, since it is predicted to be one of the most defended. In line with ODT predictions, reproductive tissues were more defended than vegetative tissues constitutively, and within the former, pistils and anthers more defended than perianths. No change in the overall defense level was found between bud maturity stages, but a significant temporal shift was observed between pistils and anthers. Contrary to ODT predictions, mechanical damage did not induce systemic defenses in leaves but only in pistils. Our results show that defense allocation in plant reproductive tissues occurs at fine spatial and temporal scales, extending the application framework of the ODT. They also demonstrate interactions between space and time in fine-scale defense allocation.

Different molecular changes underlie the same phenotypic transition: Origins and consequences of independent shifts to homostyly within species

The repeated transition from outcrossing to selfing is a key topic in evolutionary biology. However, the molecular basis of such shifts has been rarely examined due to lack of knowledge of the genes controlling these transitions. A classic example of mating system transition is the repeated shift from heterostyly to homostyly. Occurring in 28 angiosperm families, heterostyly is characterized by the reciprocal position of male and female sexual organs in two (or three) distinct, usually self-incompatible floral morphs. Conversely, homostyly is characterized by a single, self-compatible floral morph with reduced separation of male and female organs, facilitating selfing. Here, we investigate the origins of homostyly in Primula vulgaris and its microevolutionary consequences by integrating surveys of the frequency of homostyles in natural populations, DNA sequence analyses of the gene controlling the position of female sexual organs (CYPᵀ), and microsatellite genotyping of both progeny arrays and natural populations characterized by varying frequencies of homostyles. As expected, we found that homostyles displace short-styled individuals, but long-style morphs are maintained at low frequencies within populations. We also demonstrated that homostyles repeatedly evolved from short-styled individuals in association with different types of loss-of-function mutations in CYPᵀ. Additionally, homostyly triggers a shift to selfing, promoting increased inbreeding within and genetic differentiation among populations. Our results elucidate the causes and consequences of repeated transitions to homostyly within species, and the putative mechanisms precluding its fixation in P. vulgaris. This study represents a benchmark for future analyses of losses of heterostyly in other angiosperms.

A CLAVATA3-like Gene Acts as a Gynoecium Suppression Function in White Campion

How do separate sexes originate and evolve? Plants provide many opportunities to address this question as they have diverse mating systems and separate sexes (dioecy) that evolved many times independently. The classic "two-factor" model for evolution of separate sexes proposes that males and females can evolve from hermaphrodites via the spread of male and female sterility mutations that turn hermaphrodites into females and males, respectively. This widely accepted model was inspired by early genetic work in dioecious white campion (Silene latifolia) that revealed the presence of two sex-determining factors on the Y-chromosome, though the actual genes remained unknown. Here, we report identification and functional analysis of the putative sex-determining gene in S. latifolia, corresponding to the gynoecium suppression factor (GSF). We demonstrate that GSF likely corresponds to a Y-linked CLV3-like gene that is specifically expressed in early male flower buds and encodes the protein that suppresses gynoecium development in S. latifolia. Interestingly, GSFY has a dysfunctional X-linked homolog (GSFX) and their synonymous divergence (dS = 17.9%) is consistent with the age of sex chromosomes in this species. We propose that female development in S. latifolia is controlled via the WUSCHEL-CLAVATA feedback loop, with the X-linked WUSCHEL-like and Y-linked CLV3-like genes, respectively. Evolution of dioecy in the S. latifolia ancestor likely involved inclusion of ancestral GSFY into the nonrecombining region on the nascent Y-chromosome and GSFX loss of function, which resulted in disbalance of the WUSCHEL-CLAVATA feedback loop between the sexes and ensured gynoecium suppression in males.

Morphology and Molecular Phylogeny of Genus Oedogonium (Oedogoniales, Chlorophyta) from China

Oedogoniales comprises the three genera Oedogonium, Oedocladium, and Bulbochaete, which include more than 600 described species. The classification of Oedogoniaceae is currently based on morphology, and the complicated morphological characteristics make species identification difficult, with the limited molecular data also restricting the phylogenetic analysis. In the present study, we collected 47 Oedogonium specimens from China and sequenced 18S rDNA, ITS2, ITS (ITS1 + 5.8S + ITS2), and rbcL sequences to conduct phylogenetic analyses. We selected nine morphological characteristics, most of which were considered important in traditional systematics, for comparison with the molecular phylogeny results. All the topologies based on different datasets showed similar results; Oedogonium was a paraphyletic group, and Oedocladium and Bulbochaete clustered with Oedogonium. The morphological characteristics matching the phylogenetic results showed that the types of sexual differentiation, characteristics of the oogonium (including shape, types of aperture, and ornamentation of oospore wall), division types of antheridial, and number of sperm of each antheridial, which are considered the most important morphological characteristics in traditional taxonomy of Oedogonium, did not form monophyletic lineages respectively, indicating that traditional systematics may not reflect the real phylogeny of the genus Oedogonium. In addition, a new taxonomical classification of the genus Oedogonium was presented according to the shapes of basal cells, which matched well with the phylogenetic topologies. In addition, we propose to divide the genus Oedogonium into two sections, section Globosum and section Elongatum, representing the species with spherical or sub-hemispherical basal cells and elongated basal cells, respectively.

Insect herbivores drive sex allocation in angiosperm flowers

Why sex has evolved and is maintained is an open question in evolutionary biology. The Red Queen hypothesis predicts that host lineages subjected to more intense parasite pressure should invest more in sexual reproduction to continuously create novel defences against their rapidly evolving natural enemies. In this comparative study across the angiosperms, we show that hermaphrodite plant species associated with higher species richness of insect herbivores evolved flowers with higher biomass allocation towards the male sex, an indication of their greater outcrossing effort. This pattern remained robust after controlling for key vegetative, reproductive and biogeographical traits, suggesting that long-term herbivory pressure is a key factor driving the selfing-outcrossing gradient of higher plants. Although flower evolution is frequently associated with mutualistic pollinators, our findings support the Red Queen hypothesis and suggest that insect herbivores drive the sexual strategies of flowering plants and their genetic diversity.

Evolution of the sex-determining region in Ginkgo biloba

Sex chromosomes or sex-determining regions (SDR) have been discovered in many dioecious plant species, including the iconic 'living fossil' Ginkgo biloba, though the location and size of the SDR in G. biloba remain contradictory. Here we resolve these controversies and analyse the evolution of the SDR in this species. Based on transcriptome sequencing data from four genetic crosses we reconstruct male- and female-specific genetic maps and locate the SDR to the middle of chromosome 2. Integration of the genetic maps with the genome sequence reveals that recombination in and around the SDR is suppressed in a region of about 50 Mb in both males and females. However, occasional recombination does occur except a small, less than 5 Mb long region that does not recombine in males. Based on synonymous divergence between homologous X- and Y-linked genes in this region, we infer that the Ginkgo SDR is fairly old-at least of Cretaceous origin. The analysis of substitution rates and gene expression reveals only slight Y-degeneration. These results are consistent with findings in other dioecious plants with homomorphic sex chromosomes, where the SDR is typically small and evolves in a region with pre-existing reduced recombination, surrounded by long actively recombining pseudoautosomal regions. This article is part of the theme issue 'Sex determination and sex chromosome evolution in land plants'.

Selection drives convergent gene expression changes during transitions to co-sexuality in haploid sexual systems

Co-sexuality has evolved repeatedly from unisexual (dioicous) ancestors across a wide range of taxa. However, the molecular changes underpinning this important transition remain unknown, particularly in organisms with haploid sexual systems such as bryophytes, red algae and brown algae. Here we explore four independent events of emergence of co-sexuality from unisexual ancestors in brown algal clades to examine the nature, evolution and degree of convergence of gene expression changes that accompany the breakdown of dioicy. The amounts of male versus female phenotypic differences in dioicous species were not correlated with the extent of sex-biased gene expression, in stark contrast to what is observed in animals. Although sex-biased genes exhibited a high turnover rate during brown alga diversification, some of their predicted functions were conserved across species. Transitions to co-sexuality consistently involved adaptive gene expression shifts and rapid sequence evolution, particularly for male-biased genes. Gene expression in co-sexual species was more similar to that in females rather than males of related dioicous species, suggesting that co-sexuality may have arisen from ancestral females. Finally, extensive convergent gene expression changes, driven by selection, were associated with the transition to co-sexuality. Together, our observations provide insights on how co-sexual systems arise from ancestral, haploid UV sexual systems.

Mixed pollination system and floral signals of Paepalanthus (Eriocaulaceae): insects and geitonogamy ensure high reproductive success

BACKGROUND AND AIMS

Eriocaulaceae exhibit a great variety of floral traits associated with insect (e.g. nectariferous structures) and wind pollination (unisexual flowers, exposed sexual organs and small pollen grains), as well as the 'selfing syndrome' (small flowers, short distance between stigma and anthers, and temporal overlap of male and female phases). Paepalanthus bifidus, P. subtilis and P. tortilis are related species that differ in form, size and colour of floral structures. We aimed to investigate the pollination and reproductive biology of these three species.

METHODS

We analysed the floral biology, floral visitors, pollinator behaviour, and the contribution of insects, wind and spontaneous geitonogamy to fruit set. We also evaluated the floral colour and scent of the species. Colour reflectance of capitula of each species was measured and plotted in models of insect vision. Floral scent samples were extracted and the compounds were compared to vegetative scent samples.

KEY RESULTS

In all species, the staminate and pistillate flowers are arranged in alternating cycles with a temporal overlap between these phases. Ants were the most frequent floral visitors and were effective pollinators in P. bifidus and P. tortilis, while flies were occasional pollinators in P. tortilis. Floral visitors were not observed in P. subtilis. In all species, fruits were produced by spontaneous geitonogamy, with no evidence of wind pollination. According to the models of insect vision, the colours of the capitula of P. bifidus and P. subtilis are the most inconspicuous for ants and flies. We found no difference between the emission of volatiles of inflorescences and vegetative structures.

CONCLUSIONS

This study suggests that ant pollination might be more widespread in Eriocaulaceae than currently assumed. Furthermore, for small monocarpic plants, mixed mating strategies are most favourable, by ensuring reproduction either by outcrossing when pollinators are abundant or by spontaneous geitonogamy when pollinations are scarce/absent.

Do Genetic Drift and Gene Flow Affect the Geographic Distribution of Female Plants in Gynodioecious Lobelia siphilitica?

Variation in population sex ratio is particularly pronounced in gynodioecious angiosperms. Extremely high female frequencies in gynodioecious populations cannot be readily explained by selective forces alone. To assess the contributions of drift and gene flow to extreme sex-ratio variation, we documented sex ratio and population size in 92 populations of Lobelia siphilitica across its range and genotyped plants using plastid and nuclear genetic markers. Similarity in spatial patterns of genetic and demographic variables may suggest that drift and/or gene flow have contributed to population sex-ratio variation in L. siphilitica. We found strong spatial structuring of extremely high female frequencies: populations with >50% female plants are restricted to the south−central portion of the range. However, we did not detect any spatial structuring in population size nor metrics of genetic diversity, suggesting that extreme variation in female frequency is not strongly affected by drift or gene flow. Extreme sex-ratio variation is frequently observed in gynodioecious plants, but its causes are difficult to identify. Further investigation into mechanisms that create or maintain the spatial structure of sex ratios in gynodioecious species will provide much needed insight.

Genetic divergence and range expansion in a western North Pacific coral

Coral poleward range expansions have recently been observed in response to warming oceans. Range expansion can lead to reduced genetic diversity and increased frequency of deleterious mutations that were rare in core populations, potentially limiting the ability for adaptation and persistence in novel environments. Successful expansions that overcome these founder effects and colonize new habitat have been attributed to multiple introductions from different sources, hybridization with native populations, or rapid adaptive evolution. Here, we investigate population genomic patterns of the reef-building coral Acropora hyacinthus along a latitudinal cline that includes a well-established range expansion front in Japan using 2b-RAD sequencing. A total of 184 coral samples were collected across seven sites spanning from ~24°N to near its northern range front at ~33°N. We uncover the presence of three cryptic lineages of A. hyacinthus, which occupy discrete reefs within this region. Only one lineage is present along the expansion front and we find evidence for its historical occupation of marginal habitats. Within this lineage we also find evidence of bottleneck pressures associated with expansion events including higher clonality, increased linkage disequilibrium, and lower genetic diversity in range edge populations compared to core populations. Asymmetric migration between populations was also detected with lower migration from edge sites. Lastly, we describe genomic signatures of local adaptation potentially attributed to lower winter temperatures experienced at the more recently expanded northern populations. Together these data illuminate the genomic consequences of range expansion in a coral and highlight how adaptation to discrete environments along expansion fronts may facilitate further range expansion in this temperate coral lineage.

Female self-incompatibility type in heterostylous Primula is determined by the brassinosteroid-inactivating cytochrome P450 CYP734A50

Most flowering plants are hermaphrodites, with flowers having both male and female reproductive organs. One widespread adaptation to limit self-fertilization is self-incompatibility (SI), where self-pollen fails to fertilize ovules.^1,2 In homomorphic SI, many morphologically indistinguishable mating types are found, although in heteromorphic SI, the two or three mating types are associated with different floral morphologies.^3-6 In heterostylous Primula, a hemizygous supergene determines a short-styled S-morph and a long-styled L-morph, corresponding to two different mating types, and full seed set only results from intermorph crosses.^7-9 Style length is controlled by the brassinosteroid (BR)-inactivating cytochrome P450 CYP734A50,¹⁰ yet it remains unclear what defines the male and female incompatibility types. Here, we show that CYP734A50 also determines the female incompatibility type. Inactivating CYP734A50 converts short S-morph styles into long styles with the same incompatibility behavior as L-morph styles, and this effect can be mimicked by exogenous BR treatment. In vitro responses of S- and L-morph pollen grains and pollen tubes to increasing BR levels could only partly explain their different in vivo behavior, suggesting both direct and indirect effects of the different BR levels in S- versus L-morph stigmas and styles in controlling pollen performance. This BR-mediated SI provides a novel mechanism for preventing self-fertilization. The joint control of morphology and SI by CYP734A50 has important implications for the evolutionary buildup of the heterostylous syndrome and provides a straightforward explanation for why essentially all of the derived self-compatible homostylous Primula species are long homostyles.¹¹.

Switching it up: algal insights into sexual transitions

While the process of meiosis is highly conserved across eukaryotes, the sexual systems that govern life cycle phase transitions are surprisingly labile. Switches between sexual systems have profound evolutionary and ecological consequences, in particular for plants, but our understanding of the fundamental mechanisms and ultimate causes underlying these transitions is still surprisingly incomplete. We explore here the idea that brown and green algae may be interesting comparative models that can increase our understanding of relevant processes in plant reproductive biology, from evolution of gamete dimorphism, gametogenesis, sex determination and transitions in sex-determining systems.

The leaf polarity factors SGS3 and YABBYs regulate style elongation through auxin signaling in Mimulus lewisii

Style length is a major determinant of breeding strategies in flowering plants and can vary dramatically between and within species. However, little is known about the genetic and developmental control of style elongation. We characterized the role of two classes of leaf adaxial-abaxial polarity factors, SUPPRESSOR OF GENE SILENCING3 (SGS3) and the YABBY family transcription factors, in the regulation of style elongation in Mimulus lewisii. We also examined the spatiotemporal patterns of auxin response during style development. Loss of SGS3 function led to reduced style length via limiting cell division, and downregulation of YABBY genes by RNA interference resulted in shorter styles by decreasing both cell division and cell elongation. We discovered an auxin response minimum between the stigma and ovary during the early stages of pistil development that marks style differentiation. Subsequent redistribution of auxin response to this region was correlated with style elongation. Auxin response was substantially altered when both SGS3 and YABBY functions were disrupted. We suggest that auxin signaling plays a central role in style elongation and that the way in which auxin signaling controls the different cell division and elongation patterns underpinning natural style length variation is a major question for future research.

Selfing rate variation within species is unrelated to life-history traits or geographic range position

PREMISE

In plants, populations and species vary widely along the continuum from outcrossing to selfing. Life-history traits and ecological circumstances influence among-species variation in selfing rates, but their general role in explaining intraspecific variation is unknown. Using a database of plant species, we test whether life-history traits, geographic range position, or abundance predict selfing rate variation among populations.

METHODS

We identified species where selfing rates were estimated in at least three populations at known locations. Two key life-history traits (generation time and growth form) were used to predict within-species selfing rate variation. Populations sampled within a species' native range were assessed for proximity to the nearest edge and abundance. Finally, we conducted linear and segmented regressions to determine functional relationships between selfing rate and geographic range position within species.

RESULTS

Selfing rates for woody species varied less than for herbs, which is explained by the lower average selfing rate of woody species. Relationships between selfing and peripherality or abundance significantly varied among species in their direction and magnitude. However, there was no general pattern of increased selfing toward range edges. A power analysis shows that tests of this hypothesis require studying many (i.e., 40+) populations.

CONCLUSIONS

Intraspecific variation in plant mating systems is often substantial yet remains difficult to explain. Beyond sampling more populations, future tests of biogeographic hypotheses will benefit from phylogeographic information concerning specific range edges, the study of traits influencing mating system (e.g., herkogamy), and measures of abundance at local scales (e.g., population density).

Stresses affect inbreeding depression in complex ways: disentangling stress-specific genetic effects from effects of initial size in plants

The magnitude of inbreeding depression (ID) varies unpredictably among environments. ID often increases in stressful environments suggesting that these expose more deleterious alleles to selection or increase their effects. More simply, ID could increase under conditions that amplify phenotypic variation (CV²), e.g., by accentuating size hierarchies among plants. These mechanisms are difficult to distinguish when stress increases both ID and phenotypic variation. We grew in- and outbred progeny of Mimulus guttatus under six abiotic stress treatments (control, waterlogging, drought, nutrient deficiency, copper addition, and clipping) with and without competition by the grass Poa palustris. ID differed greatly among stress treatments with δ varying from 7% (control) to 61% (waterlogging) but did not consistently increase with stress intensity. Poa competition increased ID under nutrient deficiency but not other stresses. Analyzing effects of initial size on performance of outbred plants suggests that under some conditions (low N, clipping) competition increased ID by amplifying initial size differences. In other cases (e.g., high ID under waterlogging), particular environments amplified the deleterious genetic effects of inbreeding suggesting differential gene expression. Interestingly, conditions that increased the phenotypic variability of inbred progeny regularly increased ID whereas variability among outbred progeny showed no relationship to ID. Our study reconciles the stress- and phenotypic variability hypotheses by demonstrating how specific conditions (rather than stress per se) act to increase ID. Analyzing CV² separately in inbred and outbred progeny while including effects of initial plant size improve our ability to predict how ID and gene expression vary across environments.

A Draft Genome of the Ginger Species Alpinia nigra and New Insights into the Genetic Basis of Flexistyly

Angiosperms possess various strategies to ensure reproductive success, such as stylar polymorphisms that encourage outcrossing. Here, we investigate the genetic basis of one such dimorphism that combines both temporal and spatial separation of sexual function, termed flexistyly. It is a floral strategy characterised by the presence of two morphs that differ in the timing of stylar movement. We performed a de novo assembly of the genome of Alpinia nigra using high-depth genomic sequencing. We then used Pool-seq to identify candidate regions for flexistyly based on allele frequency or coverage differences between pools of anaflexistylous and cataflexistylous morphs. The final genome assembly size was 2 Gb, and showed no evidence of recent polyploidy. The Pool-seq did not reveal large regions with high FST values, suggesting large structural chromosomal polymorphisms are unlikely to underlie differences between morphs. Similarly, no region had a 1:2 mapping depth ratio which would be indicative of hemizygosity. We propose that flexistyly is governed by a small genomic region that might be difficult to detect with Pool-seq, or a complex genomic region that proved difficult to assemble. Our genome will be a valuable resource for future studies of gingers, and provides the first steps towards characterising this complex floral phenotype.

Natural selection on a carbon cycling trait drives ecosystem engineering by Sphagnum (peat moss)

Sphagnum peat mosses have an extraordinary impact on the global carbon cycle as they control long-term carbon sequestration in boreal peatland ecosystems. Sphagnum species engineer peatlands, which harbour roughly a quarter of all terrestrial carbon, through peat accumulation by constructing their own niche that allows them to outcompete other plants. Interspecific variation in peat production, largely resulting from differences in tissue decomposability, is hypothesized to drive niche differentiation along microhabitat gradients thereby alleviating competitive pressure. However, little empirical evidence exists for the role of selection in the creation and maintenance of such gradients. In order to document how niche construction and differentiation evolved in Sphagnum, we quantified decomposability for 54 species under natural conditions and used phylogenetic comparative methods to model the evolution of this carbon cycling trait. We show that decomposability tracks the phylogenetic diversification of peat mosses, that natural selection favours different levels of decomposability corresponding to optimum niche and that divergence in this trait occurred early in the evolution of the genus prior to the divergence of most extant species. Our results demonstrate the evolution of ecosystem engineering via natural selection on an extended phenotype, of a fundamental ecosystem process, and one of the Earth's largest soil carbon pools.

Reproductive development and genetic structure of the mycoheterotrophic orchid Pogoniopsis schenckii Cogn

BACKGROUND

Pogoniopsis schenckii Cogn. is a mycoheterotrophic orchid that can be used as a model to understand the influence of mycoheterotrophy at different stages of the reproductive cycle. We aimed to verify the presence of endophytic and epiphytic fungi at each stage of the reproductive process and investigated how the breeding system may relate to genetic structure and diversity of populations. In this study we performed anatomical and ultrastructural analyses of the reproductive organs, field tests to confirm the breeding system, and molecular analysis to assess genetic diversity and structure of populations.

RESULTS

During the development of the pollen grain, embryo sac and embryogenesis, no fungal infestation was observed. The presence of endophytic fungal hyphae was observed just within floral stems and indehiscent fruit. Beyond assuring the presence of fungus that promote seed germination, specific fungi hyphae in the fruit may affect other process, such as fruit ripening. As other mycoheterotrophic orchids, P. schenckii is autogamous, which may explain the low genetic diversity and high genetic structure in populations.

CONCLUSIONS

We discuss an interesting interaction: fungal hyphae in the indehiscent fruit. These fungal hyphae seem to play different roles inside fruit tissues, such as acting in the fruit maturation process and increasing the proximity between fungi and plant seeds even before dispersion occurs. As other mycoheterotrophic orchids, P. schenckii is autogamous, which may explain the low genetic diversity and high genetic structure in populations. Altogether, our findings provide important novel information about the mechanisms shaping ecology and evolution of fragmented populations of mycoheterotrophic plant.

The biomechanics of pollen release: new perspectives on the evolution of wind pollination in angiosperms

Evolutionary transitions from animal to wind pollination have occurred repeatedly during the history of the angiosperms, but the selective mechanisms remain elusive. Here, we propose that knowledge of pollen release biomechanics is critical for understanding the ecological and evolutionary processes underpinning this shift in pollination mode. Pollen release is the critical first stage of wind pollination (anemophily) and stamen properties are therefore likely to be under strong selection early in the transition. We describe current understanding of pollen release biomechanics to provide insights on the phenotypic and ecological drivers of wind pollination. Pollen release occurs when detachment forces dominate resistive forces retaining pollen within anthers. Detachment forces can be active or passive depending on whether they require energy input from the environment. Passive release is more widespread in anemophilous species and involves processes driven by steady or unsteady aerodynamic forces or turbulence-induced vibrations that shake pollen from anthers. We review empirical and theoretical studies suggesting that stamen vibration is likely to be a key mechanism of pollen release. The vibration response is governed by morphological and biomechanical properties of stamens, which may undergo divergent selection in the presence or absence of pollinators. Resistive forces have rarely been investigated for pollen within anthers, but are probably sensitive to environmental conditions and depend on flower age, varying systematically between animal- and wind-pollinated species. Animal and wind pollination are traditionally viewed as dichotomous alternatives because they are usually associated with strikingly different pollination syndromes. But this perspective has diverted attention from subtler, continuously varying traits which mediate the fluid dynamic process of pollen release. Reinterpreting the flower as a biomechanical entity that responds to fluctuating environmental forces may provide a promising way forward. We conclude by identifying several profitable areas for future research to obtain deeper insight into the evolution of wind pollination.

Size-dependent sex allocation in Solanum lycocarpum St. Hil. (Solanaceae)

Resource allocation to reproduction can change depending on size, as predicted by the size-dependent sex allocation. This theory is based on the fact that small individuals will invest in the allocation of sex with lower cost of production, usually male gender. In plants, there are some andromonoecy species, presence of hermaphrodite and male flowers in the same individual. Andromonoecy provides a strategy to optimally allocate resources to male and female function, evolving a reproductive energy-saving strategy. Thus, our objective was to investigate the size-dependent sex allocation in Solanum lycocarpum St. Hil. We tested the hypothesis that plants with larger size will invest in the production of hermaphrodite flowers, because higher individuals have greater availability of resources to invest in more complex structures involving greater energy expenditure. The studied species was S. lycocarpum, an andromonoecious species. From June 2016 to March 2017 the data were collected in 38 individuals, divided in two groups: the larger plant group (n=18; height=3-5 m) and the smaller plant group (n=20; height=1-2 m).Our data show that there was effect of plant size on the flower production and the sexual gender allocation. The larger plants showed more flowers and higher production of hermaphrodite flowers. Furthermore, in the flower scale, we observed allometric relationship among the flower's traits with proportional investments in biomass, anther size and gynoecium size. Our results are in agreement with size-dependent sex allocation theory and andromonoecy hypothesis related to mechanisms for optimal resource allocation to male and female function.

Evolution of floral scent in relation to self-incompatibility and capacity for autonomous self-pollination in the perennial herb Arabis alpina

BACKGROUND AND AIMS

The transition from outcrossing to selfing is a frequent evolutionary shift in flowering plants and is predicted to result in reduced allocation to pollinator attraction if plants can self-pollinate autonomously. The evolution of selfing is associated with reduced visual floral signalling in many systems, but effects on floral scent have received less attention. We compared multiple populations of the arctic-alpine herb Arabis alpina (Brassicaceae), and asked whether the transition from self-incompatibility to self-compatibility has been associated with reduced visual and chemical floral signalling. We further examined whether floral signalling differ between self-compatible populations with low and high capacity for autonomous self-pollination, as would be expected if benefits of signalling decrease with reduced dependence on pollinators for pollen transfer.

METHODS

In a common garden we documented flower size and floral scent emission rate and composition in eight self-compatible and nine self-incompatible A. alpina populations. These included self-compatible Scandinavian populations with high capacity for autonomous self-pollination, self-compatible populations with low capacity for autonomous self-pollination from France and Spain, and self-incompatible populations from Italy and Greece.

KEY RESULTS

The self-compatible populations produced smaller and less scented flowers than the self-incompatible populations. However, flower size and scent emission rate did not differ between self-compatible populations with high and low capacity for autonomous self-pollination. Floral scent composition differed between self-compatible and self-incompatible populations, but also varied substantially among populations within the two categories.

CONCLUSIONS

Our study demonstrates extensive variation in floral scent among populations of a geographically widespread species. Contrary to expectation, floral signalling did not differ between self-compatible populations with high and low capacity for autonomous self-pollination, indicating that dependence on pollinator attraction can only partly explain variation in floral signalling. Additional variation may reflect adaptation to other aspects of local environments, genetic drift, or a combination of these processes.

Depicting the mating system and patterns of contemporary pollen flow in trees of the genus Anadenanthera (Fabaceae)

Anadenanthera (Fabaceae) is endemic to the Neotropics and consists of two tree species: A. colubrina (Vell.) Brenan and A. peregrina (L.) Speg. This study examined the mating system and contemporary gene flow of A. colubrina (Acol) and A. peregrina (Aper) in a highly fragmented area of the Atlantic Forest to provide valuable information that informs conservation strategies. Reproductive adults from forest remnants [n _{A. colubrina} = 30 (2.7 ha), n _{A. peregrina} = 55 (4.0 ha)] and progeny-arrays (n _{A. colubrina} = 322, n _{A. peregrina} = 300) were genotyped for seven nuclear microsatellite markers. Mating system analyses revealed that A. colubrina is a mixed mating species (t_m = 0.619) while A. peregrina is a predominantly outcrossing species (t_m = 0.905). For both Anadenanthera species, high indices of biparental inbreeding were observed (Acol = 0.159, Aper = 0.216), resulting in low effective pollination neighborhood sizes. Categorical paternity analysis revealed different scales of pollen dispersal distance: the majority of crossings occurring locally (i.e., between nearby trees within the same population), with moderate pollen dispersal coming from outside the forest fragments boundaries (Acol _mp = 30%, Aper _mp = 35%). Nevertheless, pollen immigration from trees outside the populations for both species suggests that the populations are not reproductively isolated. This study highlights the importance of evaluating both mating system and contemporary gene flow for a better understanding of the biology of Anadenanthera species. This information should be considered to ensure the effective conservation and management practices of these plant species.

Diallelic self-incompatibility is the main determinant of fertilization patterns in olive orchards

Self-incompatibility (SI) in flowering plants potentially represents a major obstacle for sexual reproduction, especially when the number of S-alleles is low. The situation is extreme in the commercially important olive tree, where in vitro pollination assays suggested the existence of a diallelic SI (DSI) system involving only two groups (G1 and G2). Varieties belonging to the same SI group cannot fertilize each other, such that successful fruit production is predicted to require pollination between varieties of different groups. To test this prediction, we explored the extent to which the DSI system determines fertilization patterns under field conditions. One hundred and seventeen olive cultivars were first genotyped using 10 highly polymorphic dinucleotide Simple Sequence Repeat (SSR) markers to ascertain varietal identity. Cultivars were then phenotyped through controlled pollination tests to assign each of them to one of the two SI groups. We then collected and genotyped 1440 open pollinated embryos from five different orchards constituted of seven local cultivars with known group of incompatibility groups. Embryos genotype information were used: (i) to assign embryos to the most likely pollen donor genotype in the neighbourhood using paternity analysis, and (ii) to compare the composition of the pollen cloud genetic among recipient trees in the five sites. The paternity analysis showed that the DSI system is the main determinant of fertilization success under field open pollination conditions: G1 cultivars sired seeds exclusively on G2 cultivars, and reciprocally. No self-fertilization events were observed. Our results demonstrate that DSI is a potent force determining pollination success among varieties within olive orchards used for production. They have the potential to improve management practices by guiding the selection of compatible varieties to avoid planting orchards containing sets of varieties with strongly unbalanced SI groups, as these would lead to suboptimal olive production.

Mating system of Datura inoxia: association between selfing rates and herkogamy within populations

Plant mating system determines, to a great extent, the demographic and genetic properties of populations, hence their potential for adaptive evolution. Variation in plant mating system has been documented between phylogenetically related species as well between populations of a species. A common evolutionary transition, from outcrossing to selfing, is likely to occur under environmental spatial variation in the service of pollinators. Here, we studied two phenotypically (in floral traits) and genetically (in neutral molecular markers) differentiated populations of the annual, insect-pollinated, plant Datura inoxia in Mexico, that differ in the service of pollinators (Mapimí and Cañada Moreno). First, we determined the populations’ parameters of phenotypic in herkogamy, outcrossing and selfing rates with microsatellite loci, and assessed between generation (adults and seedlings) inbreeding, and inbreeding depression. Second, we compared the relationships between parameters in each population. Results point strong differences between populations: plants in Mapimí have, on average, approach herkogamy, higher outcrossing rate (t_m = 0.68), lower primary selfing rate (r = 0.35), and lower inbreeding at equilibrium (F_e = 0.24) and higher inbreeding depression (δ = 0.25), than the populations of Cañada. Outcrossing seems to be favored in Mapimí while selfing in Cañada. The relationship between r and F_e were negatively associated with herkogamy in Mapimí; here, progenies derived from plants with no herkogamy or reverse herkogamy had higher selfing rate and inbreeding coefficient than plants with approach herkogamy. The difference F_e–F is positively related to primary selfing rate (r) only in Cañada Moreno which suggests inbreeding depression in selfing individuals and then genetic purging. In conclusion, mating system evolution may occur differentially among maternal lineages within populations of Datura inoxia, in which approach herkogamy favors higher outcrossing rates and low levels of inbreeding and inbreeding depression, while no herkogamy or reverse herkogamy lead to the evolution of the “selfing syndrome” following the purge of deleterious alleles despite high inbreeding among individuals.

Probing the floral developmental stages, bisexuality and sex reversions in castor (Ricinus communis L.)

Castor (Ricinus communis L) is an ideal model species for sex mechanism studies in monoecious angiosperms, due to wide variations in sex expression. Sex reversion to monoecy in pistillate lines, along with labile sex expression, negatively influences hybrid seed purity. The study focuses on understanding the mechanisms of unisexual flower development, sex reversions and sex variations in castor, using various genotypes with distinct sex expression pattern. Male and female flowers had 8 and 12 developmental stages respectively, were morphologically similar till stage 4, with an intermediate bisexual state and were intermediate between type 1 and type 2 flowers. Pistil abortion was earlier than stamen inhibition. Sex alterations occurred at floral and inflorescence level. While sex-reversion was unidirectional towards maleness via bisexual stage, at high day temperatures (T_max > 38 °C), femaleness was restored with subsequent drop in temperatures. Temperature existing for 2–3 weeks during floral meristem development, influences sexuality of the flower. We report for first time that unisexuality is preceded by bisexuality in castor flowers which alters with genotype and temperature, and sex reversions as well as high sexual polymorphisms in castor are due to alterations in floral developmental pathways. Differentially expressed (male-abundant or male-specific) genes Short chain dehydrogenase reductase 2a (SDR) and WUSCHEL are possibly involved in sex determination of castor.

Comparative analysis of corolla tube development across three closely related Mimulus species with different pollination syndromes

Fusion of petals to form a corolla tube is considered a key innovation contributing to the diversification of many flowering plant lineages. Corolla tube length often varies dramatically among species and is a major determinant of pollinator preference. However, our understanding of the developmental dynamics underlying corolla tube length variation is very limited. Here we examined corolla tube growth in the Mimulus lewisii species complex, an emerging model system for studying the developmental genetics and evo-devo of pollinator-associated floral traits. We compared developmental and cellular processes associated with corolla tube length variation among the bee-pollinated M. lewisii, the hummingbird-pollinated Mimulus verbenaceus, and the self-pollinated Mimulus parishii. We found that in all three species, cell size is non-uniformly distributed along the mature tube, with the longest cells just distal to the stamen insertion site. Differences in corolla tube length among the three species are not associated with processes of organogenesis or early development but are associated with variation in multiple processes occurring later in development, including the location and duration of cell division and cell elongation. The tube growth curves of the small-flowered M. parishii and large-flowered M. lewisii are essentially indistinguishable, except that M. parishii tubes stop growing earlier at a smaller size, suggesting a critical role of heterochrony in the shift from outcrossing to selfing. These results not only highlight the developmental process associated with corolla tube variation among species but also provide a baseline reference for future developmental genetic analyses of mutants or transgenic plants with altered corolla tube morphology in this emerging model system.

Genetic assessment of the effects of self-fertilization in a Lilium L. hybrids using molecular cytogenetic methods (FISH and ISSR)

Self-fertilization (also termed selfing) is a mode of reproduction that occurs in hermaphrodites and has evolved several times in various plant and animal species. A transition from outbreeding to selfing in hermaphroditic flowers is typically associated with changes in flower morphology and functionality. This study aimed to identify genetic effects of selfing in the F2 progeny of F1 hybrid developed by crossing Lilium lancifolium with the Asiatic Lilium hybrid ‘Dreamland.’ Fluorescence in situ hybridization (FISH) and inter-simple sequence repeats (ISSR) techniques were used to detect genetic variations in plants produced by selfing. The FISH results showed that F1 hybrid were similar to the female parent (L. lancifolium) regarding the 45S loci, but F2 individuals showed variation in the number and location of the respective loci. In F2 progeny, F2-2, F2-3, F2-4, F2-5, and F2-8 hybrids expressed two strong and one weak 5S signal on chromosome 3, whereas F2-7 and F2-9 individuals expressed one strong and two weak signals. Only two strong 5S signals were detected in an F2-1 plant. The ISSR results showed a maximum similarity value of 0.6269 between the female parent and the F2-2 hybrid. Regarding similarity to the male parent, a maximum value of 0.6119 was found in the F2-1 and F2-2 hybrids. The highest genetic distance from L. lancifolium and the Asiatic Lilium hybrid ‘Dreamland’ was observed in the F2-4 progeny (0.6352 and 0.7547, respectively). Phylogenetic relationships showed that the F2 progeny were closer to the male parent than to the female parent. Self-fertilization showed effects on variation among the F2 progeny, and effects on the genome were confirmed using FISH and ISSR analyses.

What Do We Really Know About Adaptation at Range Edges?

Recent theory and empirical evidence have provided new insights regarding how evolutionary forces interact to shape adaptation at stable and transient range margins. Predictions regarding trait divergence at leading edges are frequently supported. However, declines in fitness at and beyond edges show that trait divergence has sometimes been insufficient to maintain high fitness, so identifying constraints to adaptation at range edges remains a key challenge. Indirect evidence suggests that range expansion may be limited by adaptive genetic variation, but direct estimates of genetic constraints at and beyond range edges are still scarce. Sequence data suggest increased genetic load in edge populations in several systems, but its causes and fitness consequences are usually poorly understood. The balance between maladaptive and positive effects of gene flow on fitness at range edges deserves further study. It is becoming increasingly clear that characterizations about degree of adaptation based solely on geographical peripherality are unsupported.

Gynomonoecy in a mycoheterotrophic orchid Eulophia zollingeri with autonomous selfing hermaphroditic flowers and putatively outcrossing female flowers

Most orchid species exhibit an extreme case of hermaphroditism, owing to the fusion of male and female organs into a gynostemium. Exceptions to this rule have only been reported from the subtribes Catasetinae and Satyriinae. Here, I report an additional orchidaceous example whose flowers are not always hermaphroditic. In several Japanese populations of Eulophia zollingeri (Rchb.f.) J.J.Sm, a widespread Asian and Oceanian orchid, some flowers possess both the anther (i.e., anther cap and pollinaria) and stigma, whereas others possess only the stigma. Therefore, pollination experiments, an investigation of floral morphology and observations of floral visitors were conducted to understand the reproductive biology of E. zollingeri in Miyazaki Prefecture, Japan. It was confirmed that E. zollingeri studied here possesses a gynomonoecious reproductive system, a sexual system in which a single plant has both female flowers and hermaphroditic flowers. In addition, hermaphroditic flowers often possess an effective self-pollination system while female flowers could avoid autogamy but suffered from severe pollinator limitation, due to a lack of agamospermy and low insect-mediated pollination. The present study represents the first documented example of gynomonoecy within Orchidaceae. Gynomonoecy in E. zollingeri may be maintained by the tradeoff in reproductive traits between female flowers (with low fruit set but potential outcrossing benefits) and hermaphroditic flowers (with high fruit set but inbreeding depression in selfed offspring). This mixed mating is probably important in mycoheterotrophic E. zollingeri because it occurs in shaded forest understorey with a paucity of pollinators.

Supergene evolution via stepwise duplications and neofunctionalization of a floral-organ identity gene

Heterostyly represents a fascinating adaptation to promote outbreeding in plants that evolved multiple times independently. While l-morph individuals form flowers with long styles, short anthers, and small pollen grains, S-morph individuals have flowers with short styles, long anthers, and large pollen grains. The difference between the morphs is controlled by an S-locus "supergene" consisting of several distinct genes that determine different traits of the syndrome and are held together, because recombination between them is suppressed. In Primula, the S locus is a roughly 300-kb hemizygous region containing five predicted genes. However, with one exception, their roles remain unclear, as does the evolutionary buildup of the S locus. Here we demonstrate that the MADS-box GLOBOSA2 (GLO2) gene at the S locus determines anther position. In Primula forbesii S-morph plants, GLO2 promotes growth by cell expansion in the fused tube of petals and stamen filaments beneath the anther insertion point; by contrast, neither pollen size nor male incompatibility is affected by GLO2 activity. The paralogue GLO1, from which GLO2 arose by duplication, has maintained the ancestral B-class function in specifying petal and stamen identity, indicating that GLO2 underwent neofunctionalization, likely at the level of the encoded protein. Genetic mapping and phylogenetic analysis indicate that the duplications giving rise to the style-length-determining gene CYP734A50 and to GLO2 occurred sequentially, with the CYP734A50 duplication likely the first. Together these results provide the most detailed insight into the assembly of a plant supergene yet and have important implications for the evolution of heterostyly.

Evolution of self-incompatibility in the Brassicaceae: Lessons from a textbook example of natural selection

Self-incompatibility (SI) is a self-recognition genetic system enforcing outcrossing in hermaphroditic flowering plants and results in one of the arguably best understood forms of natural (balancing) selection maintaining genetic variation over long evolutionary times. A rich theoretical and empirical population genetics literature has considerably clarified how the distribution of SI phenotypes translates into fitness differences among individuals by a combination of inbreeding avoidance and rare-allele advantage. At the same time, the molecular mechanisms by which self-pollen is specifically recognized and rejected have been described in exquisite details in several model organisms, such that the genotype-to-phenotype map is also pretty well understood, notably in the Brassicaceae. Here, we review recent advances in these two fronts and illustrate how the joint availability of detailed characterization of genotype-to-phenotype and phenotype-to-fitness maps on a single genetic system (plant self-incompatibility) provides the opportunity to understand the evolutionary process in a unique perspective, bringing novel insight on general questions about the emergence, maintenance, and diversification of a complex genetic system.

The specialization–generalization continuum in oil-bee pollination systems: a case study of six Brazilian species of Tigridieae (Iridaceae)

Several South American species of Iridaceae, especially those of Tigridieae, produce floral oils as rewards to oil-bee pollinators. The present study aimed to contribute to a deeper understanding of the reproductive biology, pollination ecology and level of specialization of the interactions of species encompassed in Tigridieae. Data on breeding and pollination systems were acquired from six species native to Southern Brazil. The visitation frequency and pollen load of pollen- and oil-collecting bees were also investigated. The results strongly suggest that the studied species are distributed along a specialization–generalization continuum. Three oil-producing taxa, Cypella herbertii, Cypella pusilla and Cypella amplimaculata, were pollinated effectively by oil-bees, whereas in the other two studied species, Kelissa brasiliensis and Herbertia pulchella, the oil-bees appeared to function as oil thieves, owing to failure to contact the plant reproductive parts during oil-foraging behaviour. New insights into aspects of the specialization–generalization continuum of pollination systems, differences in pollinator behaviour during oil and pollen foraging, and reproductive outputs of the studied species are provided. Taken together, our results provide a significant contribution towards a better understanding of reproductive biology and plant–pollinator interactions between Iridaceae and oil-collecting bees.

Hybridization and geographic distribution shapes the spatial genetic structure of two co-occurring orchid species

Multiple ecological and life-history traits shape the fine-scale spatial genetic structure (FSGS) of a given population. The occurrence in core versus peripheral populations, levels of outcrossing, pollen and seed dispersal, and hybridization are important biological properties that influence the kinship of individuals within populations. We examined spatial genetic structure within 15 populations of Epidendrum fulgens and E. puniceoluteum distributed along a linear gradient of Brazilian coastal vegetation, including both allopatric and sympatric populations where the two orchid species hybridize. We analyzed 581 mapped specimens using nine simple sequence repeat loci, aiming to investigate how geographic distribution and hybridization shape within-population FSGS. A significant increase in FSGS was found towards peripheral populations, compared to core populations. Analysis of short-distance and long-distance components of FSGS identified biparental inbreeding and higher levels of FSGS at peripheral populations, when compared to core populations. In contrast, the relatively high density of reproductive adults in core populations potentially leads to highly overlapping seed and pollen movement, decreasing FSGS. Hybridization was an important factor shaping within-population spatial genetic structure at sympatric sites, decreasing the FSGS observed in parental species. Our results indicate that different ecological forces act in concert to create a gradient of FSGS along species distribution ranges, shaped by extensive levels of intraspecific and interspecific gene exchange.

Comparative development of staminate and pistillate flowers in the dioecious cactus Opuntia robusta

PCD role in unisexual flowers. The developmental processes underlying the transition from hermaphroditism to unisexuality are key to understanding variation and evolution of floral structure and function. A detailed examination of the cytological and histological patterns involved in pollen and ovule development of staminate and pistillate flowers in the dioecious Opuntia robusta was undertaken, and the potential involvement of programmed cell death in the abortion of the sex whorls was explored. Flowers initiated development as hermaphrodites and became functionally unisexual by anthesis. Female individuals have pistillate flowers with a conspicuous stigma, functional ovary, collapsed stamens and no pollen grains. Male individuals have staminate flowers, with large yellow anthers, abundant pollen grains, underdeveloped stigma, style and an ovary that rarely produced ovules. In pistillate flowers, anther abortion resulted from the premature degradation of the tapetum by PCD, followed by irregular deposition of callose wall around the microsporocytes, and finally by microspore degradation. In staminate flowers, the stigma could support pollen germination; however, the ovaries were reduced, with evidence of placental arrest and ovule abortion through PCD, when ovules were present. We demonstrate that PCD is recruited in both pistillate and staminate flower development; however, it occurs at different times of floral development. This study contributes to the understanding of the nature of the O. robusta breeding system and identifies developmental landmarks that contribute to sexual determination in Cactaceae.